I don't know the rules for this but it does come up often among airline pilots I have met.
My recollection, which may be worth checking out, is that if your ATPL is a CAA-issued one then you get automatic IMC Rating privileges for a SE plane.
CAA CPL/ATPL holders get this and it's been around for years. I don't know if you have to apply for the IMCR or whether it is automatic.
If it is a JAA-issued one then you don't get this, although I have read somewhere that somebody managed to get the CAA to give him an IMCR anyway.
I know for certain that a CAA or JAA ATPL (multi crew, one always assumes) does not give you a SE IR - that has to be done with a checkride at least. A friend has just done this.